1. Field of the Invention
This invention relates to an optical disk, an apparatus for reproducing information from optical disk and method of recording/reproducing information on or from an optical disk.
2. Description of the Prior Art
FIG. 1 illustrates construction of a known optical disk on which audio information and/or video information is recorded. Information is converted into signals using pit lengths as parameters, and the signals are recorded on information recording surface 95 as information pits 91. In reproduction, a laser light is converged on information pits 91 to form a light spot LS having predetermined diameter, and reflected and diffracted light beam is detected by photodetector, such as photodiode, to produce electric signal. The electric signal is subjected to a conversion reverse to the conversion in recording to obtain audio information or video information recorded on the disk 92. As examples of such an optical disk, Compact Disk (CD) and Laser Disk (LD) are known. In FIG. 1, reference numeral 93 indicates substrate and reference numeral 94 indicates protection layer.
As described above, as to a known optical disk, reading light beam is irradiated on disk and recorded information is read out based on variation of quantity of reflected light beam resulting from presence or absence of information pits. Information recording density of those optical disks are limited by track pitch. As shown in FIG. 2, track pitch P.sub.1 is an interval between neighboring tracks which define center-lines of columns of information pits 91. Recently, in order to increase recording density of optical disk, various measures for reduction of track pitch are developed.
However, when track pitch is simply reduced to value P.sub.2 smaller than conventional value P.sub.1 (e.g., P.sub.2 = P.sub.1 /2) as illustrated in FIG. 3, light spot LS of laser light covers not only information pit 91A to be read-out but also information pits 91B and 91C on neighboring tracks. This trial is hopeless in practice because reproduced signal includes crosstalk (intermixture of signals from neighboring tracks) too much. Namely, in order to reduce crosstalk, track pitch must be large enough compared with a diameter of light spot LS. This is a bar against increasing recording density.
In the above view, in order to improve recording density, various methods for reducing diameter of light spot of laser beam has been proposed. In general, radius W of light spot LS is expressed as follows: EQU W=.alpha..times..lambda./NA (.alpha.:constant).
Here, NA is numerical aperture of objective lens, and .lambda. is wavelength of laser light. Accordingly, in order to reduce diameter W of laser light, wavelength .lambda. should be reduced, or alternatively numeral aperture NA should be increased. However, wavelength .lambda. may be reduced to be half at the utmost, and NA may be increased 10% at the utmost. Therefore, according to this method, it is hopeless to improve recording density greatly.
On the other hand, methods for improving recording density has been proposed in which multiple information are expressed by unique information pit (Japanese Patent Application No. 03-187746, U.S. Patent Application Ser. No. 07/998,056, now U.S. Pat. No. 5,359,591, issued on Oct. 25, 1994). According to these methods, if light spot is minimized, recording density is further improved. However, in the above-mentioned methods, since shapes of information pits are recognized by pattern recognition utilizing diffraction phenomenon of light beam, affection from neighboring information pits and deviation of relative position of light spot is relatively large. Accordingly, it is difficult to sufficiently improve recording density.